The invention relates to the technical field of semiconductors, and discloses a split-gate MOS device and a preparation method thereof. The split-gate MOS device comprises a substrate, a first epitaxial layer, a second epitaxial layer and a third epitaxial layer of a first
conduction type are sequentially arranged on the substrate from bottom to top, the
doping concentration of the second epitaxial layer is greater than that of the first epitaxial layer, the
doping concentration of the first epitaxial layer is larger than that of the second epitaxial layer, the
doping concentration of the second epitaxial layer is larger than that of the third epitaxial layer, and the first epitaxial layer, the second epitaxial layer and the third epitaxial layer form a drift region of the split-gate MOS device. Firstly, the second epitaxial layer with high doping concentration can enhance the
electric field modulation effect of the drift region and increase the withstand
voltage of the split-gate MOS device; secondly,
electric field lines can be prevented from being accumulated on two sides of the bottom of the groove, and the split-gate MOS device is prevented from being broken down on two sides of the bottom of the groove in advance when withstanding
voltage; and thirdly, the on-resistance of the device is reduced.